The present invention relates to barium titanate particles with surface deposition of a rare earth element such as yttrium or, more particularly, to barium titanate particles bearing a water-insoluble compound of a rare earth element such as oxides or water-insoluble salts of the rare earth element as well as to a method for the preparation of such barium titanate particles.
As is well known, powders of barium titanate are widely employed long since as a material of ceramic capacitors. It is also known and practiced to admix the barium titanate powder with a compound of a rare earth element such as a rare earth oxide or, in particular, yttrium oxide in the preparation of barium titanate-based ceramic capacitors with an object to improve performance of the ceramic capacitor. A typical procedure of admixing a rare earth compound to a barium titanate powder is that a rare earth oxide powder is blended with the barium titanate powder and the powder blend is thoroughly agitated in a suitable powder-blending machine such as ball mills to give a uniform mixture of barium titanate particles and rare earth oxide particles usable in the preparation of ceramic capacitors.
In compliance with the trend in recent years in the field of electronic technology that ceramic capacitors are required to be more and more compact in size or smaller and smaller in thickness, the above mentioned powder blend of barium titanate particles and rare earth oxide particles is also required to have increased uniformity of blending in addition to the requirement for a smaller and smaller particle diameter of the barium titanate particles as well as of the rare earth oxide particles. Since the rare earth oxide powder products available on the market usually have an average particle diameter exceeding the sub-micron range and a highly uniform powder blend can hardly be obtained from such a rare earth oxide powder with a barium titanate powder, full improvement of the performance of thin-layered or compact-size ceramic capacitors cannot be accomplished by the admixture of a rare earth oxide powder to a barium titanate powder.
The present invention accordingly has an object, in order to overcome the above described problems in the prior art, to provide a powder based on barium titanate particles containing a rare earth compound with microscopically high uniformity of mixing.
Thus, the present invention provides barium titanate particles bearing a water-insoluble salt or an oxide of a rare earth element deposited on the surface of the particles.
The invention also provides a method for the preparation of the above defined barium titanate-based particles which comprises the steps of:
(a) dispersing particles of barium titanate in an aqueous solution as a suspension medium containing a water-soluble salt of a rare earth element and urea dissolved therein to give an aqueous suspension;
(b) heating the aqueous suspension of barium titanate particles prepared in step (a) at a temperature of 70xc2x0 C. or higher to effect hydrolysis of urea and to deposit a basic carbonate of the rare earth element on the surface of the barium titanate particles; and
(c) separating the barium titanate particles having surface deposition of the basic carbonate of the rare earth element from the suspension medium.
The basic carbonate of the rare earth element deposited on the surface of the barium titanate particles can be converted, in step (d), to a rare earth oxide by calcining the powder at a temperature of 600xc2x0 C. or higher in an oxidizing atmosphere to give a barium titanate powder containing a rare earth oxide with high uniformity.